1. Field of the Invention
The present invention relates to fruit- and nut-harvesting equipment, and more particularly to a compliant engagement member for use with the heads of tree-shaking apparatus.
2. Description of the Related Art
Mechanical devices for harvesting fruit and nuts from trees typically operate by gripping the tree trunk and imparting thereto a controlled, low-frequency vibration or shaking action. This operation dislodges the fruit or nuts, which are then collected and transported.
A typical tree-shaking apparatus includes two opposing clamp members that engage the tree trunk, and which are themselves driven by dual oscillation units. These units coact to rock the tree laterally. In older devices, the clamps were provided with simple pads that would make direct contact with the tree. This approach, unforunately, resulted in frequent injury to the tree due to abrasion.
To reduce the abrasive effect of pad contact, equipment manufacturers have experimented with various types of deformable engagement pads, such as those described in U.S. Pat. No. 3,318,629. Deformable pads were, for example, filled with a loose packing of granular material, and could therefore fit snugly around a variety of irregularly shaped trees or limbs without excessive local pressures. This type of configuration, while less harmful to the trees, is far less efficient at transmitting vibration than non-deformable pads, since the loosely packed material inherently absorbs vibratory force before it reaches the tree.
Current tree-shaking devices often utilize non-deformable shaker pads that contain associated lubrication arrangements; these avoid tree damage through deliberate slippage within the devices rather than against the trees. An example of this approach appears in U.S. Pat. No. 4,921,073, which describes a shaking head that includes a main shaker pad, a sling that surrounds the pad, and a slip pad that overlies the sling and makes contact on its outer face with the tree. A heat-resistant lubricant or grease is applied between the sling and the slip pad, ensuring that vibration causes the sling to slide across the inner face of the slip pad, while the outer face of the slip pad remains stationary with respect to the tree and therefore does not abrade the bark.
Although externally lubricated arrangements such as this avoid tree damage while preserving adequate transmission of vibration from the oscillation units to the tree, they require cumbersome manual or mechanical systems that replenish the lubricant as it is squeezed out from between the padding layers during operation. Like any forced-fluid device, these mechanical systems require maintenance and are vulnerable, particularly in the vigorous operating environment of a tree-shaking device, to malfunction. Furthermore, because spent lubricant is ejected or falls to the ground during operation, the possibility of environmental contamination must also be considered and prevented.
We disclosed an improvement over these designs in U.S. Pat. No. 5,406,780. Our approach involved a pouch fillable with a lubricant and configured to resist deformation along the axis of vibration, thereby avoiding dissipation of vibrational energy, while allowing sufficient off-axis deformation to prevent slippage and consequent abrasion of the tree. The disclosed engagement member included a shaker pad coupled to the frame of a vibration device; a sealed, lubricant-filled pouch rigidly attached to the pad and frame; and a contact blanket draped over the pouch.
While highly useful under a wide range of conditions, this arrangement has nonetheless been found to exhibit certain limitations. The fact that the pouch is sealed requires a fixed volume of lubricant; as a result, the lubricant may become stale from repeated use, and may also be present in greater quantity than is actually necessary. Furthermore, the contact force between the pouch (via the blanket) and the tree can be controlled only by adjusting the frame; because the frame is a large device not amenable to convenient, precise movement, in practical effect the contact force simply cannot be finely controlled. But insufficient contact between the pouch and the tree prevents efficient transmission of vibratory force and can allow the pouch to strike the tree (risking its injury), while excessive contact force also risks improper operation and injury to the tree.
The rigid connection between the pouch and the frame can also prove problematic. Although the inner surfaces of the pouch are free to slip relative to one another, movement of the frame during vibration can exceed the allowable slippage, with the result that the pouch (again, via the blanket) cannot remain fixed against the tree; instead the pouch drags against the tree, abrading the bark.